U.S. patent number 6,491,421 [Application Number 09/726,784] was granted by the patent office on 2002-12-10 for fluid mixing system.
This patent grant is currently assigned to Schlumberger Technology Corporation. Invention is credited to Joel Rondeau, Pierre Vigneaux.
United States Patent |
6,491,421 |
Rondeau , et al. |
December 10, 2002 |
Fluid mixing system
Abstract
A method for continuously mixing a borehole fluid such as cement
includes using a measurement of the solid fraction of a cement
slurry as it is being mixed to determine the ratio of the solid and
liquid components to be added to the slurry. A system for mixing
the includes a liquid material (water) supply including a flow
meter; a solid material (cement) supply; a mixer which receives the
liquid and solid materials and includes an output for delivering
materials from the mixer to a delivery system; a device for
measuring the amount of material in the mixer; and a flow meter in
the output; wherein measurements from the flow meters and the
device for measuring the amount of material in the mixer are used
to control the amount of solid and/or liquid material added to the
mixer.
Inventors: |
Rondeau; Joel (Antony,
FR), Vigneaux; Pierre (Moisenay, FR) |
Assignee: |
Schlumberger Technology
Corporation (Sugar Land, TX)
|
Family
ID: |
24920002 |
Appl.
No.: |
09/726,784 |
Filed: |
November 29, 2000 |
Current U.S.
Class: |
366/8; 366/136;
366/18; 366/17; 366/141; 366/153.1 |
Current CPC
Class: |
B01F
3/1271 (20130101); G05D 11/135 (20130101); B01F
15/00123 (20130101); B01F 15/00136 (20130101); B01F
15/00149 (20130101); B01F 15/00155 (20130101); B01F
15/00194 (20130101); B01F 15/00207 (20130101); B01F
15/0022 (20130101); B01F 15/00233 (20130101); B01F
15/00344 (20130101); B01F 15/0445 (20130101); B28C
7/022 (20130101); E21B 21/062 (20130101); E21B
33/13 (20130101); G05D 11/132 (20130101); B01F
5/106 (20130101); B01F 2215/0047 (20130101); B01F
2215/0081 (20130101); B01F 2215/0409 (20130101) |
Current International
Class: |
B01F
5/00 (20060101); B01F 15/00 (20060101); B01F
5/10 (20060101); E21B 21/06 (20060101); E21B
21/00 (20060101); B28C 7/00 (20060101); B28C
7/02 (20060101); B01F 3/12 (20060101); E21B
33/13 (20060101); G05D 11/00 (20060101); G05D
11/13 (20060101); B28C 007/04 () |
Field of
Search: |
;366/136,137,152.2,152.1,152.6,141,153.1,18,17,8 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
195 12 416 |
|
Apr 1995 |
|
DE |
|
0 403 283 |
|
Dec 1990 |
|
EP |
|
Primary Examiner: Soohoo; Tony G.
Attorney, Agent or Firm: Schlather; Stephen F. Menes;
Catherine Jeffery; Brigitte
Claims
What is claimed:
1. A system for mixing a cement slurry in a well cementing
operation, comprising: i) a liquid material supply including means
for controlling the flow of liquid therefrom, and a first flow
meter for determining the flow rate of liquid supplied therefrom;
ii) a solid cement supply including means for controlling the flow
of solid cement supplied therefrom; iii) a mixer which receives the
liquid and solid cement from the liquid material supply and solid
cement supply respectively and mixes them to form a slurry, and
includes an output for delivering materials from the mixer, a
second flow meter being located in the output for determining the
flow rate of slurry from the mixer; iv) a device for determining
the variation over time of the amount of slurry in the mixer; v) a
delivery system connected to the output of the mixer for delivering
the slurry to a well; and vi) a monitoring system which determines
the ratio of solid cement and liquid in the mixer from flow rates
determined by the first and second flow meters and from the
determined variation over time of the amount of slurry in the
mixer; wherein the means for controlling the flow of liquid and
means for controlling the flow of solid cement are operated to
control the relative amounts of solid and liquid material added to
the mixer according to the determined ratio of solid cement and
liquid in the mixer.
2. A system as claimed in claim 1, wherein the liquid material
supply includes water and the solid material supply includes cement
and other solid materials.
3. A system as claimed in claim 2, wherein the supply of solid
cement comprises separate supplies of cement and dry additives, a
third flow meter being provided to measure the rate of flow of the
dry additives to the mixer.
4. A system as claimed in claim 3, wherein the supply of dry
additives comprises multiple separate supplies of additives, each
with its own flow meter.
5. A system as claimed in claim 1, wherein the monitoring system
determines the solid fraction of the slurry in the mixer in order
to determine the ratio of solid cement and liquid in the mixer, the
means for controlling the flow of solid cement being operated
according to the determined solid fraction to control the amount of
solid cement added to the mixer.
6. A system as claimed in claim 1, wherein the flow meters are
selected from mass flow meters and volumetric flow meters.
7. A system as claimed in claim 1, wherein the flow meters are
selected from Coriolis meters and electromagnetic meters.
8. A system as claimed in claim 1, wherein the mixer comprises a
mixing section, a mixing tub, a feeder which feeds slurry from the
mixing section to the mixing tub, and recirculation system which
recirculates a portion of the slurry from the tub to the mixing
section.
9. A system as claimed in claim 8, wherein the device for measuring
the variation over time of the amount of slurry in the mixer
measures the variation over time of the amount of slurry in the
tub.
10. A system as claimed in claim 9, wherein the device comprises a
level sensor in the tub.
11. A system as claimed in claim 9, wherein the device comprises a
load sensor which measures the weight of the tub over time so as to
determine the variation over time of the amount of slurry
therein.
12. A system as claimed in claim 8, wherein the recirculation
system is located upstream of the second flow meter located in the
output of the mixer.
Description
FIELD OF THE INVENTION
The present invention relates to a system for mixing fluids
containing solid and liquid materials such as cement. In particular
the invention provides a system for the continuous mixing of
cements or other fluids used in the drilling, completion or
stimulation of boreholes such as oil or gas wells.
BACKGROUND OF THE INVENTION
When a well such as an oil or gas well has been drilled, it is
often desired to isolate the various producing zones form each
other or from the well itself in order to stabilise the well or
prevent fluid communication between the zones or shut off unwanted
fluid production such as water. This isolation is typically
achieved by installing a tubular casing in the well and filling the
annulus between the outside of the casing and the wall of the well
(the formation) with cement. The cement is usually placed in the
annulus by pumping a slurry of the cement down the casing such that
it exits at the bottom of the well and passes back up the outside
of the casing to fill the annulus. While it is possible to mix the
cement as a batch prior to pumping into the well, it has become
desirable to effect continuous mixing of the cement slurry at the
surface just prior to pumping into the well. This has been found to
provide better control of cement properties and more efficient use
of materials.
The cement slurries used in such operations comprise a mixture of
dry and liquid materials. The liquid phase is typically water and
so is readily available and cheap. The solid materials define the
slurry and cement properties when added to the water and mixed, the
amount of solid materials in the slurry being important. Since the
liquid phase is constant, the amount of solid material added is
usually monitored by measuring the density of the slurry and
maintaining this at the desired level by controlling the amount of
the solid material being added. FIG. 1 shows a schematic diagram of
a prior art mixing system. In the system of FIG. 1, mix water is
pumped from a feed supply 10 via a pump 12 to a mixer 14 which
feeds into a mixing tub 16. Solid materials are delivered to the
mixer 14 from a surge can 18 via a flow control valve 20 and are
carried into the mixing tub 16 with the mix water. The contents of
the mixing tub 16 are recirculated through a recirculation pipe 22
and pump 24 to the mixer 14. The recirculation pipe 22 also
includes a densitometer 26 which provides a measurement of the
density of the slurry in the mixing tub 16. An output 28 is
provided for slurry to be fed from the mixing tub 16 to further
pumps (not shown) for pumping into the well. Control of the slurry
mixture is achieved by controlling the density in the mixing tub 16
as provided by the densitometer 26 by addition of solid material to
stay at a predetermined level for the slurry desired to be pumped.
The densitometer 26 is typically a non-radioactive device such as a
Coriolis meter.
While this system is effective for slurries using materials of much
higher density than water, it is not effective for slurries using
low density solid materials, especially when the density of the
solids is close to that of water. In such cases, a density
measurement is not sensitive enough to control the amounts of solid
material added to the necessary accuracy.
The present invention seeks to provide a mixing system which avoid
the problem of density measurement described above.
SUMMARY OF THE INVENTION
In its broadest aspect, the present invention comprises using a
measurement of the solid fraction of a fluid as it is being mixed
to determine the ratio of the solid and liquid components added to
the slurry.
The invention is particularly applicable to the mixing of borehole
cement slurries, in which case, solids fraction is determined as
(slurry vol-water vol)/slurry vol. An alternative but related
parameter is porosity, determined as water vol/slurry vol
(porosity+solids fraction=1).
A system for mixing cement in accordance with the invention
comprises a water supply including a flow meter; a solid material
supply; a mixer which receives the water and solid materials and
includes an output for delivering materials from the mixer to a
delivery system; a device for measuring the amount of material in
the mixer; and a flow meter in the output; wherein measurements
from the flow meters and the device for measuring the amount of
material in the mixer are used to control the amount of solid
material added to the mixer.
The flow meters can be mass flow meters or volumetric flow meters.
Any suitable form of meter can be used, for example Coriolis meters
or electromagnetic meters.
The mixer will typically include a tank or tub, in which case the
device for measuring the amount of material in the mixer can be a
level sensor. Such a level sensor is preferably a time domain
reflectometry- or radar-type device although acoustic or float
devices can also be used. It is preferred to mount such a device in
an arrangement for damping transient fluctuations in the tank
level, for example in an arrangement of concentric slotted tubes.
An alternative or additional form of sensor can be a load cell
which can be used to indicate the weight of the tank, or a pressure
sensor.
Where the mixer includes some form of recirculation of the slurry
through the tank, it is important that the output flow meter is
downstream of this recirculation.
Where the solid materials comprise cement and other solid additives
added separately to the mixer, separate flow meters can also be
provided for each separate supply of additives.
In its simplest form, the measurement of solid fraction is used as
a guide for the operator to add solids, particularly cement, to the
slurry as it is mixed. In more advanced versions, the calculation
of solids fraction is used to control the addition of solids
directly by means of an automatic control system.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a prior art mixing system;
FIG. 2 shows a mixing system according to a first embodiment of the
invention;
FIG. 3 shows the components of a tank level sensor;
FIG. 4 shows the components of the level sensor assembled;
FIG. 5 shows a schematic of the tank level measurement; and
FIG. 6 shows a mixing system according to a second embodiment of
the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The system shown in FIG. 2 is used for the continuous mixing of
cement for oil well cementing operations and comprises a supply of
mix water 100 feeding, via a pump 102 and a flow meter 104 to a
mixing system 106. The mixing system 106 also receives solid
materials from a surge can 108 which are admitted through a valve
110. The mixed solid and liquid materials are delivered through a
feed pipe 112 to a mixing tub 114. The mixing tub 114 has a first
outlet 116 connected to a recirculation pump 118 which feeds the
slurry drawn from the tub 114 back into the mixing system. The tub
114 is provided with a level sensor 120 and/or a load sensor 122 to
provide an indication of the tank contents and any change in
contents over time. A second output 124' is provided from the tub
114 which leads, via a second pump 126 and a second flow meter 128'
to the pumping system from which it is delivered to the well (not
shown). An alternative method of delivery (shown in dashed line in
FIG. 2) has an output 124' taken from the recirculation line via a
flow meter 128' to the well. Other arrangements are also possible.
The pumps 102, 118, 126 are of the usual type found in well
cementing systems, for example centrifugal pumps. Likewise, the
flow meters 104, 128' are conventional, for example Coriolis meters
such as those that have been used as densitometers in previous
applications. Different types of pumps and meters each have
advantages and disadvantages that are well known in the art and can
be selected according to requirements.
FIGS. 3-5 show details of the tub level sensor and its
installation. The sensor comprises a Krohne radar sensor 200, a
stainless steel rod 202, an inner slotted sleeve 204 and an outer
slotted sleeve 206. The rod 202 is screwed onto the sensor 200 and
the inner sleeve 204 mounted over the rod 202 and attached to a
flange on the sensor 200. The outer sleeve 206 is mounted over the
inner sleeve 204 to which it is attached. In use, the sensor
arrangement is installed in the mixing tub 114 in the vertical
position and in a location where the slurry is renewed as the
mixing occurs, to avoid location in a dead zone where cement might
set. The sensor provides a measurement of the difference between
the length of the rod 202 (LM) and the level of slurry in the tub
level (TL). The free tub level (FTL) is obtained by:
FTL=LM-TL.
It will be appreciated that the exact form of level sensor is not
important to the overall effect of the invention. What is important
is to obtain an indication of the variation versus time of the tub
slurry volume (called "tub flow" in this document). This can be
obtained using a float or a load sensor or combinations of any of
these or any other sensor giving this information.
The outputs of the flow sensors and level sensors are used to
monitor the solid fraction of the slurry in the following
manner:
The solid fraction computation is based on the balance between
incoming and outgoing volumes (or flow rates) as expressed in the
following relationship:
where Q.sub.tub is the tub rate.
Tub rate is the variation versus time of the tub volume and is
considered as positive while the tub level increases and negative
while it decreases. The smaller the tub cross section, the more
sensitive the measurement will be to change. Q.sub.tub is given by:
##EQU1##
where S.sub.tub is the tub cross section and ##EQU2##
is the tub level variation over time. In the simplest case, the tub
section is constant and the tub rate be comes the product of the
tub level variation/time and the tub cross section.
The solids fraction at time t is computed as the ratio of (slurry
vol-water vol) over the total slurry volume present at time t in
the tub. The variation in tub slurry volume V.sub.tub
(t+.delta.t)-V.sub.tub (t) can be expressed as:
which can be rewritten as:
In the same way, the variation in the water volume present in the
tub at time t V.sub.water (t+.delta.t)-V.sub.water (t) is equal to
the incoming water volume minus the amount of water present in the
slurry leaving the tub, and can be expressed as:
Solid Fraction is then expressed as: ##EQU3##
The calculation requires that the initial conditions be known if it
is to be accurate ab initio, i.e. is the tub empty, full of water
or containing slurry already. The calculation will ultimately
stabilise independently of the initial conditions, the time taken
to do this depending on the tub volume and the output flow rate
Q.sub.slurry.
These calculations are conveniently performed using a computer, in
which case the measurements can be provided directly from the
sensors via a suitable interface. A preferred screen display will
show the various flow rates or levels, together with the desired
solids fraction (calculated when designing the slurry). The mixing
process is controlled by adjusting the amount of cement and/or
water added to the mixer so as to maintain the calculated solids
fraction at the desired level. Alternatively, the results of the
calculations can be fed to an automatic control system which
adjusts the rate at which the components are delivered to the
mixing system.
The system described above works well when the dry ingredients
(blend of cement+additives) are delivered pre-mixed to the well
site from another location. In this case essentially the same
measurements and calculations as described above are performed,
merely substituting Q.sub.blend for Q.sub.cement. If it is desired
to mix the dry materials on site as part of the continuous mixing
process, a slightly different approach is required. FIG. 6 shows a
mixing system according to another embodiment of the invention and
uses a numbering scheme which follows that of FIG. 2. The system of
FIG. 6 comprises an additional dry material supply 130 which admits
the dry products to the mixing system 106 via a mass flow meter 132
(other flow measurement means can also be used) and a control valve
134. In this case, the basic control equation becomes:
where four of the five variables are know and Q.sub.cement is the
most difficult parameter to measure accurately. Where multiple dry
additives are to be added, the supply can comprise separate
material supplies, each with a flow meter and valve. Additional
terms Q.sub.additive1, Q.sub.additive2, etc., are included in the
control equation.
It will be appreciated that changes can be made in implementation
while still remaining within the scope of using solid fraction as
the property monitored to effect control of the mixing.
For example, the method can be applied to the mixing of other
borehole fluids such as stimulation fluids (fracturing fluids) or
even drilling fluids (mud). In the case of fracturing fluids, the
gel and proppant (liquid and solid phases) are usually mixed using
a pod blender and the proportion of gel and proppant controlled
using a densitometer (usually radioactive) downstream of the
mixer/blender. The use of radioactive sensors generates many
environmental issues and while Coriolis-type meters are an
alternative, they are know to have limitations in respect of flow
rate when used this way. The present invention allows control of
proppant and gel concentrations by means of flow meters without the
need to rely on densitometer measurements.
Gel and mixed fluid flow rates are measured by means of
electromagnetic flow meters. The amount of proppant is directly
deduced from the following relationship:
Proppant concentration (in Pounds Per Gallon Added or "PPA") can be
a function of solid fraction as defined above and expressed as the
following:
Thus the solid fraction measurement methodology described above in
relation to cement can be applied to fracturing fluids by
determining proppant density rather than cement density.
This approach has the advantage of not requiring the use of
radioactive densitometers thus avoiding limitations placed on use
for regulatory reasons and without the flow rate performance
limitations of other measurement techniques. The equipment and
control system is essentially the same as that used in the
cementing system described above.
* * * * *